follow up to schema changes

- update XMLVMachineData class to reflect mandatory status of <vmdisk> configuration setting in XML
  + previously the <vmdisk> element was optional and it was possible to
    construct the VMachine object without arguments. Now the <vmdisk> element
    is mandatory and thus the object may no longer be constructed without
    argument.

- also did some style clean up of the test while I was at it ;)